Process for coating titanium articles



United. States. Patent No Drawing. Filed Oct. 27, 1958, Ser. No. 769,5691 Claim. (Cl. 204-37) This invention relates to a process-for finishingarticles made of titanium.

Titanium is a relatively new metal of commerce. It is strong and lightin weight. As a structural material it gives evidence of beinga modernsuccessor of aluminum for many purposes. It is difiicult, however, topolish mechanically or by chemical or electrochemical techniques; andalloyed titanium structural members that are subjected to stress, inparticular, are susceptible to corrosion, particularly salt corrosion.Moreover, titanium and its alloys have a tendency to gall and seize whenin moving contact with other metals; and this restricts their utility.

One object of the invention is to provide a process for applying adurable, protective and decorative coating to titanium and itscommercial alloys.

Another object of the invention is to provide a process for producing acoating for titanium and its alloys that is resistant to abrasion, andcorrosion.

Another object of the invention is to provide a process for treatingtitanium and its alloys to produce on it a surface coating that has alubricating quality.

Another object of the invention is to produce a coating for titanium andits alloys that is of uniform thickness and color.

A related objectof the invention is to provide a process for producing auniform, tough coating of substantial thickness on articles made oftitanium or its alloys, where their fabrication or use demands freedomfrom seizure or galling.

Other objects of the invention will be apparent hereinafter from thespecification and from the recital of theappended claim.

We have found that a hard, opaque, non-porous, coating can be formed onunalloyed titanium, crystal-bar titanium, and titanium alloys by a highvoltage anodizing process. In general, the process is performed byimmersing the titanium article in an acidic or alkaline bath of acharacter that will not attack the titanium to an undesirable extentduring anodizing, and making the article the anode in an electricalcircuit. A voltage of about at least 1300 volts and up to about 2000volts is then applied. The thickness of the coating can be regulated bycontrolling the voltage and the time during which the voltage isapplied. The quality of appearance is somewhat affected, however, by thedegree of chemical purity and of physical uniformity of the commercialtitanium.

The process can be practiced at low voltage when thin anodic coatingsare desired. Coating thickness does not exceed approximately 40 Angstromunits per volt of applied potential regardless of time.

In one embodiment of the invention, a sheet of commercially puretitanium is cleaned to remove all foreign substance from its surface;and the plate is immersed in an anodizing bath containing a solution ofboric acid saturated at room temperature. Preferably a stainless steeltank is used as the container and cathode. The tank is immersed in astoneware jar that contains circulating coolant to maintain thetemperature of the bath near 32 F. Some acid precipitates from thesolution at this operating temperature.

The stainless steel tank is grounded. The titanium plate or sheet isconnected terminaLto function as the anode, and a high voltage is in thecircuit to the. positive applied to it. Example: a sample of unalloyedtitanium anodized in boric acid as described above at 1500-voltsacquired a 0.25-mil thickcoatingpas determined by metallographicexamination of a tapered cross-section.

The voltage is usually raised gradually in increments of 200 volts up to1500 to 2000 volts. It is also possible to raise the voltage rapidly topeak value. When the full voltage is built up in 200-volt increments,instead ofbeing applied instantly, the resulting anodic coatings aremore adherent.

The anode current density varies from 15 amp. per sq. ft. to 30 amp. persq. ft. when peak voltage is attained. It decreases with time to about1.5 amp. per sq. ft. as

the anodic coating builds up to maximum thickness, about 0.1 mil to 0.2mil. It has been found that the temperature of the bath and the shape ofthe plate or panel influence the current density corresponding to agiven voltage. The current density increases with rising bathtemperature, and is higher on panelswith a larger edge length-to-surfacearea ratio than on other panels of the same area; Where the edgelength-to-surface area is very large it is neces sary to round off sharpedgesto avoid corona discharge at the edges.

The coatings have proved to be very adherent to un-:.

alloyed titanium. They do not chip or powder off when a sheet is given asharp bend. Only a minute amount of theanodic film is loosened as a finepowder at the bend, but except for this negligible powdering, thecoating'remains on the metal and is not removed even by hard rubbingwith a mild abrasive material. The tough,

adherentcoating is especially useful where seizure and galling areproblems.

The followingtitanium alloys have been successfullyanodized by thesubject process: Ti-6Al-4V, Ti-4.5Al-12Zr, Ti-7Al-4Mo, Ti-4Al-3Mo-1V,and Ti-5Al-2.5Sn.

yellow to yellow-golden color.

against stress corrosion. Example: When roundtensile bars of Ti-6Al-4V,Ti-4.5Al-l2Zr, and Ti-7Al-4Mo were anodized in the boric acid bath up tonear 1500 volts in 200 to 250-volt increments, the bars, withoutexceptions were not damaged by salt stress corrosion within 50 hours at750 F. and 45 to 65 k.s.i. creep stress.

To control the thickness of the coating obtained, the peak voltage andthe anodizing time are controlled. In general, a coating thickness onthe order of at least 0.1 mil is required for practical resistance toabrasion. Titanium, that has been subjected to high voltage anodizationto produce a hard non-porous coating having a substantially uniformthickness of about 0.1 mil, is eminently suitable in the manufacture ofstructural members and panels, as well as for other purposes.

In general, it is preferred to carry out the anodizing in a weaklyacidic bath, at temperatures below about F. in order to retard acidattack on the metal. Saturated solutions of boric acid, or diluteammonium salt solutions of tartaric acid, and citric acid are eminentlysuitable.

Anodizing can also be efifected at low voltages either with a stronglyacid bath or a strongly alkaline bath. Sulphuric or chromic acids, orsalts of these acids, such as potassium and lithium sulphates, have beenemployed, as have also baths containing potassium and lithiumhydroxides. At low voltages, the films formed are thin and clear. Thefilm thickness and color are controlled by cell voltage. Above aspecific voltage (50 to volts, or more, for strongly acid baths, and 25to 50 volts for the alkaline baths tested) these thin films break downPatented Jan. 29, 1963* Coat-z ings on the vanadium-containingalloyssometimes have a None of alloys takes on coatings'which are soadherent asthe coating obtained dielectrically with arcing. Thebreakdown voltages depend, of course, upon the bath composition. We havefound, however, that if the baths contain lithium ions, they can beoperated above the normal breakdown voltage. The presence of lithiumions in, the bath promotes uniform anodic coatings on titanium. This isapparently due to the ability of the lithium ion to penetrate thecoating and provide for conductivity in the TiO film. In a fifty percentKOH bath, for instance, coating started at the edges of a panel or sheetand worked toward the center. Adding one percent LiOH to the bathallowed coating to start forming simultaneously all over the surface ofthe panel or sheet. A saturated (fifteen percent) solution of LiOHallowed a panel to be coated at high rate at very low current density(200-700 volts) without arcing. Even at high current densities no arcingoccurred. Instead of lithium hydroxide minute amounts of lithium borateor lithium tartrate may be employed in the bath.

The hard, opaque, non-porous coatings formed by highvoltage anodizingaccording to the process of the invention provide a marked contrast tothe coatings that are obtained through low voltage anodization when nolithium compound is used in the bath. At low voltages (but high enoughto cause dielectric breakdown), the anodized coatirigs may be grey,light green, or light brown, and are absorbent, and porous. The coatingsproduced by the high voltage anodizing process of this invention aresubstantially uniform and tough.

High voltage coatings, being non-porous, cannot be dyed. Coated partscan be colored, however, by heating the coated parts in a tube furnacein an atmosphere of commercially available argon. The specimen is placedin a tube furnace and the furnace is brought to temperature and held attemperature for a suitable period. Then the specimen is furnace-cooled.Under these conditions, some of the coating oxide diffuses into themetal, and the rest of the oxide remains as a semi-transparent and veryabrasion-resistant cover over the temper color. Commercial argoncontains some nitrogen as an impurity, and it is believed, the nitrogenmay be responsible for coloring the titanium. Two of the colors that canbe obtained are yellow and blue. Other colors can be obtained by controlover the temperature and holding time in the furnace.

The titanium base metal recrystallizes at temperatures above about 1625F. Therefore, when a titanium part,

that has a coating formed thereon by high voltage anodizing, issubjected to a temperature above 1625 F. in the process of coloring it,randomly oriented crystal faces are formed on the base metal, and thecolored layer between the metal and its coating appears spangled becauseof its location on the randomly oriented crystal faces. This effect isvery pleasing in appearance. At temperatures below 1625 F., the originalreflectance and luster of the coated part is substantially retained.

While the invention has been described in connection with specificembodiments thereof, then, it will be understood that it is capable offurther modification, and this application is intended to cover anyvariations, uses, or adaptations of the invention following, in general,the principles of the invention and including such departures from thepresent disclosure as come within known or customary practice in the artto which the invention pertains and as may be applied to the essentialfeatures hereinbefore set forth and as fall within the scope of theinvention or the limits of the appended claim.

Having thus described our invention, what we claim is:

A process for producing a hard, substantially uniform,corrosion-resistant Spangled coating on an article of titanium ortitanium alloy, comprising anodizing said arti cle in a boric acid bathat a voltage in excess of 1300 volts and at a temperature below F., andthen heating said anodized article in an atmosphere of commercial argonhaving a small, reactive amount of nitrogen, at a temperature of atleast 1625 F. and then furnace-cooling said article to producecoloration.

References Cited in the file of this patent UNITED STATES PATENTS1,925,307 De Boer et al Sept. 5, 1933 2,368,688 Taylor Feb. 6, 19452,631,115 Fox Mar. 10, 1953 2,711,496 Ruben June 21, 1955 2,804,410Wyatt Aug. 27, 1957 FOREIGN PATENTS 874,047 Germany Apr. 20, 1953 OTHERREFERENCES Chemical Abstracts, vol. 48 (1954), pages 13,489i and13,490a; abstract of publication by Tajima et a1.

